TW202028315A - Polyester film and preparation method thereof - Google Patents

Abstract

The present disclosure relates to a polyester film and a preparation method of the same. Since the polyester film includes a resin layer formed from a mixture containing a polyester resin, which includes a first diol moiety derived from isosorbide and a second diol moiety derived from cyclohexanedimethanol in a controlled ratio, and polyethylene terephthalate in a specific ratio, it is possible to exhibit improved heat resistance and adhesion with excellent light transmittance.

Description

Polyester film and its preparation method

Cross references to related applications

This application requires the Korean patent application No. 2019-0006446 filed with the Korean Intellectual Property Office on January 17, 2019, and the Korean patent application No. 2019-0080466 filed with the Korean Intellectual Property Office on July 4, 2019. Rights, the disclosure of which is incorporated into this article by reference in its entirety. This disclosure relates to a polyester film and a preparation method thereof.

As a representative example of polyester resins, polyethylene terephthalate (PET) has been widely used as optical films, electrical insulating films, due to its low price and excellent mechanical/chemical/electrical properties. Packaging film, laminated film and various protective film materials. However, PET has poor heat resistance. Therefore, a method of increasing heat resistance through a heat setting process at high temperature is used to prepare films using PET. However, when the PET film thus prepared is exposed to high temperatures for a long time, there is a problem that oligomers precipitate on the surface of the film to crystallize, and therefore the transparency of the film deteriorates. To prevent this, a method of adding a separate process such as coating has been proposed. However, there are still problems of complicated preparation methods, defects in post-processing and easy pollution. Generally, in order to improve productivity, a molding process such as printing is performed on the film at a high temperature of about 80°C. However, since PET has a low glass transition temperature of 80°C or lower, when a molding process such as printing is performed at a high temperature, the probability of occurrence of defects increases significantly. In addition, when PET has low chemical resistance to solvents used for printing, the transparency of the film may deteriorate and surface defects may occur. In addition, PET exhibits high crystallinity, especially in biaxial stretching, and therefore has disadvantages in heat sealability. Therefore, even in the high temperature process, the PET film for optics needs to have high transparency through low content of oligomers. In addition, the PET film for printing needs to have high heat resistance and chemical resistance in order to exhibit excellent productivity. In particular, for industrial or packaging applications, further research is needed on polyester films with excellent adhesion and heat sealability by controlling crystallinity.

The present disclosure provides a polyester film with excellent light transmittance and improved heat resistance and adhesion.

This disclosure also provides a method for preparing the above polyester film.

According to an embodiment of the present disclosure, a polyester film is provided, which includes a resin layer, and the resin layer is composed of polyethylene terephthalate in a weight ratio of 90:10 to 10:90, and Polyester resin (polyester resin) mixture is formed, The polyester resin has a repeating structure of an acid moiety derived from dicarboxylic acid or its derivatives and a diol moiety derived from diol, and the polyester resin Is characterized by the content (a) of the first diol part derived from isosorbide and the content (a) of the first diol part derived from isosorbide relative to the total amount of the diol part derived from the diol being 100 mole percent (mol%) The content (b) of the second diol part of cyclohexanedimethanol satisfies the following formula 1.

[Formula 1]

b≤18 mole percentage-a

Wherein, in the above formula, a is the content (mol%) of the first diol part derived from isosorbide, which is 100 mol% relative to the total amount of the diol part derived from the diol in the polyester resin, a is 4 mol% to 18 mol%, and b is the content of the second diol moiety derived from cyclohexanedimethanol.

According to another embodiment of the present disclosure, a method for preparing the above polyester film is provided.

The polyester film of the present disclosure can exhibit improved heat resistance and adhesion, and has excellent light transmittance. Therefore, polyester films can be used in various applications, such as industrial films, food container films, packaging films, optical films, insulating films, printing films, and adhesive films.

The polyester film and its preparation method according to specific embodiments of the present disclosure will be described below.

The terms used herein are only used for the purpose of describing specific embodiments, and are not intended to limit the present disclosure. The singular form also includes the plural form, unless the context clearly dictates otherwise. The terms "include", "include", etc. in the present disclosure are used to designate certain features, regions, integers, steps, operations, elements, and/or elements, and do not exclude certain other features, regions, integers, steps, operations, elements And/or the presence or addition of elements.

In addition, unless otherwise indicated in this disclosure, room temperature (RT) refers to 20±5°C.

According to an embodiment of the present disclosure, a polyester film is provided, including: a polyester film made of polyethylene terephthalate (resin A); and a polyester film different from polyethylene terephthalate (resin A); A resin layer formed by a mixture of polyester resin (resin B), wherein the weight ratio of polyethylene terephthalate to polyester resin ranges from 90:10 to 10:90, The polyester resin (resin B) has a repeating structure of an acid moiety derived from dicarboxylic acid or its derivatives and a diol moiety derived from diol, And the polyester resin is characterized by the content of the first diol part derived from isosorbide (isosorbide) relative to the total amount of the diol part derived from the diol being 100 mole percent (mol%) a) The content (b) of the second diol part derived from cyclohexanedimethanol (cyclohexanedimethanol) satisfies the following formula 1.

[Formula 1]

b≤18 mole percentage--a

(In Formula 1, a is the content (mol%) of the first diol part derived from isosorbide, and is 100 mol% relative to the total amount of the diol part derived from the diol in the polyester resin (mol%), a is 4 mol% to 18 mol%, and b is the content of the second diol part derived from cyclohexanedimethanol).

A representative example of polyester resin is polyethylene terephthalate (PET), which has low heat resistance, which limits its application. In addition, due to the high crystallinity, the adhesion may be reduced during the heat sealing process.

In order to solve these problems, a method of introducing isosorbide into the polymer backbone of the current polyester resin has been proposed. The residue derived from isosorbide can reduce the regularity of the polymer chain, thereby reducing the crystallinity of the resin. In order to ensure sufficient heat resistance and adhesion, the polyester resin should include a large amount of diol moieties derived from isosorbide. However, a large amount of the diol moiety derived from isosorbide causes a problem that it cannot be used as a crystalline resin. Amorphous resin cannot be stretched and molded due to the low regularity of its molecular structure. Therefore, there is a limit on the content of isosorbide that can be incorporated into the polymer backbone of the polyester resin.

In order to overcome these technical limitations, the present disclosure uses a specific ratio of polyethylene terephthalate (resin A) and polyester resin (resin B) to prepare an improved light transmittance, heat resistance and adhesion A polyester film, wherein the polyester resin (resin B) includes a diol part (first diol part) derived from isosorbide (ISB) and a second diol part derived from cyclohexanedimethanol (CHDM) in a controlled ratio Alcohol part (second diol part). Therefore, the polyester film can be used for optical films that require high transparency, food container films or printing films that require excellent heat resistance, and adhesive films and packaging films that require high adhesion.

Specifically, in the polyester film according to an embodiment of the present disclosure, polyethylene terephthalate (resin A) may include: through polycondensed terephthalic acid (terephthalic acid) and ethylene glycol (ethylene terephthalic acid) glycol) obtained polyethylene terephthalate; or PET-based copolymer polyester resin (PET-based copolymer polyester resin), in which a part of terephthalic acid is replaced by another dicarboxylic acid, or ethylene dicarboxylic acid The alcohol is replaced by another diol. Specifically, examples of other glycols that replace part of ethylene glycol may include neopentyl glycol, 1,4-cyclohexanedimethanol, propylene glycol, and ethylene glycol. Diol (tetramethylene glycol) and so on.

Polyethylene terephthalate (resin A) is different from the polyester resin (resin B) described below, and more specifically, may have repeated two diols derived from a diol other than isosorbide as another diol The structure of the alcohol part.

The physical properties of polyethylene terephthalate (resin A) can affect the properties of polyester resin films. Among various physical properties, the melting point can affect the heat resistance of the polyester resin film. The melting point of polyethylene terephthalate (resin A) can be adjusted by controlling the type and content of monomers and polycondensation conditions. The polyethylene terephthalate (resin A) measured using differential scanning calorimetry (DSC) may have a melting point ranging from 200°C to 260°C or 225°C to 245°C. By having a melting point in the above-mentioned temperature range, polyethylene terephthalate can further improve heat resistance in the case where a film is prepared by blending with a polyester resin (resin B) as described below.

In the polyester film according to an embodiment of the present disclosure, the polyester resin (resin B) is different from polyethylene terephthalate (resin A). The polyester resin has a repeating structure of an acid moiety derived from a dicarboxylic acid or its derivative and a diol moiety derived from a diol, and relative to the total amount of the diol moiety being 100 mol%, the poly The ester resin includes 18 mol% of the first diol moiety derived from isosorbide, and optionally includes the second diol moiety derived from cyclohexanedimethanol. When the polyester resin (resin B) further includes a second diol moiety derived from cyclohexanedimethanol, the second diol moiety is included in the content that satisfies the condition of Formula 1 above.

Specifically, the polyester resin (resin B) is based on the esterification reaction or transesterification of a dicarboxylic acid or its derivative and a diol containing isosorbide and optionally cyclohexanedimethanol. It is obtained by polycondensation reaction after transesterification reaction. Therefore, the polyester resin has a repeating structure of the acid part derived from dicarboxylic acid or its derivative and the diol part derived from diol, and the diol part derived from isosorbide and cyclohexanedimethanol is included in In the range that satisfies the above formula 1 conditions. That is, the polyester resin may include 4 to 18 mol% of the first diol part derived from isosorbide (in this case, the first diol part derived from cyclohexane The content of the diol part of alkane dimethanol is 0 mol%); or includes 4 to 18 mol% of the first diol part derived from isosorbide, and more than 0 and 14 mol% or less derived from cyclohexane In the second diol part of the alkane dimethanol, the total content of the first diol part and the second diol part is 18 mol% or less relative to the total amount of the diol part being 100 mol%.

In this disclosure, the acid moiety and the diol moiety refer to the residue left after polymerizing dicarboxylic acid or its derivatives and diol and removing hydrogen, hydroxyl group or alkoxy group from it Things.

The first diol moiety derived from isosorbide (1,4:3,6-dianhydroglucitol (1,4:3,6-dianhydroglucitol)) is introduced into the polyester resin (resin B) to reduce the crystallization rate . However, when the content is excessive, specifically, when the content exceeds 18 mol% relative to the total amount of diol parts constituting the resin as 100 mol%, the compatibility with polyethylene terephthalate may be Lowered to increase the haze, the crystallinity may be significantly reduced to make stretching and heat setting difficult. When the content of the first diol part derived from isosorbide is less than 4 mol% relative to the total amount of the diol part being 100 mol%, it is difficult to sufficiently improve heat resistance and adhesion, and turbidity may occur. The polyester resin (resin B) used in the present disclosure includes the content range of 4 to 18 mol% or 5 to 16 mol% derived from isosorbide relative to the total amount of the diol part being 100 mol% The first diol part of the alcohol therefore has improved heat resistance and adhesion, while maintaining excellent light transmittance during film preparation.

In addition, the polyester resin (resin B) may further include a second diol moiety derived from cyclohexane dimethanol whose content satisfies the above formula 1. Specifically, it may include the second diol part in the content, so that relative to the total amount of the diol part being 100 mol%, the second diol part and the first diol part derived from isosorbide The total content is 18 mol% or less.

The second diol moiety derived from cyclohexanedimethanol can be incorporated into the polyester resin to increase the light transmittance of the polyester resin. However, when the content exceeds a certain level, the amorphousness of the polyester resin may increase, so that stretching orientation may be impossible, so that there is a risk of breakage during the stretching process. Therefore, the second diol fraction derived from cyclohexanedimethanol may be included in the residual content so that relative to the total amount of the diol fraction of 100 mol%, the first diol fraction derived from isosorbide The total content is 18 mol% or less while meeting the content condition of the first diol part. Specifically, relative to the total amount of the diol part being 100 mol%, the second diol part derived from cyclohexanedimethanol may contain 14 mol% or less, 10 mol% or less, or 5 mol% % Or less, or may not be included (0 mol%).

Specific examples of cyclohexane dimethanol may include 1,2-cyclohexanediol (1,2-cyclohexanediol), 1,4-cyclohexanediol (1,4-cyclohexanediol), 1,2-cyclohexanediol Alkane dimethanol (1,2-cyclohexanedimethanol), 1,3-cyclohexanedimethanol (1,3-cyclohexanedimethanol), 1,4-cyclohexanedimethanol (1,4-cyclohexanedimethanol), etc. The polyester resin (resin B) may include a diol moiety derived from any one or a mixture of two or more of them.

With respect to the total amount of the diol part being 100 mol%, except for the diol part (first and second diol part) derived from isosorbide and cyclohexanedimethanol, the polyester resin (resin B) It may also include 82 to 96 mol% or 84 to 95 mol% of the diol moiety (third diol moiety) derived from C2 to C12 aliphatic diol. The third diol part derived from the aliphatic diol can increase the light transmittance of the polyester resin, reduce the haze, and improve the adhesion. However, when the content exceeds 96 mol%, adhesiveness may not be exhibited, and when the content is less than 82 mol%, the haze may increase.

Specific examples of aliphatic diols may include linear or branched aliphatic diols, such as ethylene glycol, diethylene glycol, triethylene glycol, propanediol (1 , 2-propanediol, 1,3-propanediol, etc.), 1,4-butanediol (1,4-butanediol), pentanediol (pentanediol), hexanediol (1,6-hexanediol, etc.) ), and neopentyl glycol (2,2-dimethyl-1,3-propanediol), and any one or two of them or A variety of mixtures. Among them, a third diol moiety derived from ethylene glycol may be included, which can further increase the light transmittance. According to an embodiment of the present disclosure, relative to the total amount of the diol part in the polyester resin, the content of the diol part derived from diethylene glycol may be 1 mol% or less, more specifically 0 mol% , Or greater than 0 mol% and 1 mol% or less. When the content of the diol moiety derived from diethylene glycol in the polyester resin is controlled within the above range, the heat resistance can be further improved.

In addition to the aforementioned diol parts (first to third diol parts) derived from isosorbide, cyclohexanedimethanol, and aliphatic diols, the polyester resin (resin B) may also include derived The diol part (fourth diol part) from other diols, for example C7 to C12 alicyclic diols.

More specifically, relative to the total amount of the diol part being 100 mol%, the diol part in the polyester resin consists of the first diol part derived from isosorbide and the second diol part derived from cyclohexanedimethanol. The diol part and the third part derived from an aliphatic diol are composed. The first diol part and the second diol part may be included in the content satisfying Formula 1 above. More specifically, relative to the total amount of the diol part being 100 mol%, the diol part in the polyester resin may be 4 to 18 mol% or 5 to 16 mol% of the first diol derived from isosorbide Part and 82 to 96 mol% or 84 to 85 mol% of the third diol part derived from ethylene glycol.

The term "dicarboxylic acid or its derivatives" as used herein refers to one or more compounds selected from dicarboxylic acids and their derivatives. The term "dicarboxylic acid derivative" refers to the alkyl ester of dicarboxylic acid (lower alkyl ester with 1-4 carbon atoms, such as monomethyl, monoethyl, Dimethyl (dimethyl, diethyl, or dibutyl ester) or anhydride of dicarboxylic acid. Therefore, for example, terephthalic acid or its derivatives are commonly referred to as compounds that react with diols to form terephthaloyl moiety, such as terephthalic acid; terephthaloyl monoalkyl ester ( monoalkyl terephthalate) or dialkyl terephthalate; and terephthalic acid anhydride.

In the polyester film according to an embodiment of the present disclosure, the polyester resin (resin B) includes an acid moiety derived from a dicarboxylic acid or its derivative, and the aforementioned diol moiety, wherein the dicarboxylic acid or its derivative may be Phthalic acid or its derivatives. Specifically, terephthalic acid or its derivatives can be used alone as dicarboxylic acid or its derivatives. In addition, dicarboxylic acid or its derivatives can be obtained by combining terephthalic acid or its derivatives with C8 to C14 aromatic dicarboxylic acid or its derivatives and C4 to C12 aliphatic dicarboxylic acids ( Aliphatic dicarboxylic acid) or at least one of its derivatives is mixed and used as a dicarboxylic acid or its derivatives other than terephthalic acid or its derivatives. Examples of C8 to C14 aromatic dicarboxylic acids or their derivatives may include aromatic dicarboxylic acids or their derivatives commonly used in the preparation of polyester resins, such as naphthalene dicarboxylic acid such as isophthalic acid ( isophthalic acid, dimethyl isophthalate, phthalic acid, dimethyl phthalate, phthalic acid anhydride and 2, 6-naphthalene dicarboxylic acid (2,6-naphthalene dicarboxylic acid), and dialkylnaphthalene dicarboxylate such as dimethyl 2,6-naphthalene dicarboxylate, Diphenyl dicarboxylic acid and so on. The C4 to C12 aliphatic dicarboxylic acid or its derivative may be a linear, branched or cyclic aliphatic dicarboxylic acid or its derivative which is generally used for preparing polyester resins. Examples thereof may include cyclohexanedicarboxylic acid (cyclohexanedicarboxylic acid) such as 1,4-cyclohexane dicarboxylic acid (1,4-cyclohexane dicarboxylic acid) and 1,3-cyclohexanedicarboxylic acid (1,3- cyclohexane dicarboxylic acid), cyclohexane dicarboxylate such as dimethyl 1,4-cyclohexane dicarboxylate and 1,3-cyclohexane dicarboxylate Methyl 1,3-cyclohexane dicarboxylate, sebacic acid, succinic acid, isodecylsuccinic acid, maleic acid, maleic anhydride anhydride), fumaric acid, adipic acid, glutaric acid, azelaic acid, etc.

The dicarboxylic acid or its derivative may preferably be terephthalic acid, dimethyl terephthalate, or a mixture of terephthalic acid and isophthalic acid among the above compounds, to ensure that the polyester resin (resin B ) Physical properties and improve the resin film.

Relative to the total amount of dicarboxylic acid or its derivatives as 100 mol%, dicarboxylic acid or its derivatives may include 40 mol% or more, 50 mol% or more, 60 mol% or more, 70 mol% or more, 80 mol% or more, or 90 mol% or more of terephthalic acid or its derivatives. With respect to the total amount of dicarboxylic acid or its derivatives as 100 mol%, dicarboxylic acid or its derivatives may include 60 mol% or less and greater than 0 mol in addition to terephthalic acid or its derivatives. % And 60 mol% or less, 0.1 to 55 mol%, 0.1 to 20 mol%, or 5 to 10 mol% of another dicarboxylic acid or its derivative. A polyester resin having appropriate physical properties can be prepared within the content range.

Since the effect can be significantly improved by controlling the type and content of the acid part and the glycol part constituting the polyester resin, the acid part can be derived from terephthalic acid or The first acid moiety of its derivative may be composed of 40 mol% or more and 100 mol% or less, or 90 to 95 mol% derived from the first acid moiety of terephthalic acid or its derivatives, and More than 0 mol% and 60 mol% or less, or 5 to 10 mol% are composed of a second acid moiety derived from a C8 to C14 aromatic dicarboxylic acid or a derivative thereof. Under the condition that the above formula 1 is satisfied, relative to the total amount of the diol part being 100 mol%, the diol part can be from 4 to 18 mol% of the first diol part derived from isosorbide, 14 mol% Or less of the second diol moiety derived from cyclohexanedimethanol and 82 to 96 mol% of the third diol moiety derived from aliphatic diols.

In addition, the polyester resin (resin B) may further include at least one or more additives, such as polycondensation catalysts, stabilizers, coloring agents, crystallization aids (polycondensation catalysts) added during the production process. crystallizing agent), antioxidant (antioxidant) or branching agent (branching agent). Specifically, based on the central metal atom, the polyester resin may further include a polycondensation catalyst selected from 1 ppm to 300 ppm, a phosphorus stabilizer from 10 ppm to 5000 ppm, and a cobalt-based coloring agent from 1 ppm to 300 ppm. at least one of the group consisting of 1 ppm to 200 ppm crystal promoter, 10 ppm to 500 ppm antioxidant, and 10 ppm to 300 ppm branching agent. Specific examples and their contents will be described in detail in the preparation method of polyester resin below.

The polyester resin (resin B) having the composition as described above can control physical properties by controlling the type and content of monomers and polymerization conditions, so as to exhibit more excellent effects when applied to resin films. Specifically, the polyester resin (resin B) may have an intrinsic viscosity of 0.50 deciliters/gram (dl/g) to 1.00 dl/g, which is a polyester resin (resin B) dissolved in a concentration of 1.2 g at 150°C /Deciliter (g/dl) of ortho-chlorophenol (ortho-chlorophenol) measured after 15 minutes at 35 ℃ intrinsic viscosity.

The intrinsic viscosity (IV) of the polyester resin (resin B) may affect the processing characteristics and mechanical strength characteristics in film preparation. The intrinsic viscosity less than the above range may cause poor appearance during the molding process due to rapid flow, and may not ensure sufficient mechanical strength. In addition, it may be difficult to obtain the desired physical properties through high stretching. In addition, when the intrinsic viscosity exceeds the above range, the pressure of the extruder increases due to the increase in the viscosity of the molten material during the molding process, so the co-extrusion process may be unstable. When the temperature of the extruder is increased to reduce the pressure, the color and physical properties may be deteriorated due to thermal deformation, and process problems may occur due to the difference in shrinkage from the base layer during stretching and heat treatment.

More specifically, the polyester resin (resin B) may have an intrinsic viscosity (or melting intrinsic viscosity) of 0.45 to 0.65 dl/g or 0.50 to 0.60 dl/g, in which the esterification reaction or the transesterification reaction and the polycondensation reaction The polymer obtained immediately thereafter was dissolved in o-chlorophenol at a concentration of 1.2 g/dl at 150°C for 15 minutes, and the intrinsic viscosity was measured at 35°C. In addition, after the esterification reaction or the transesterification reaction and the polycondensation reaction, the intrinsic viscosity can be further increased through an additional crystallization process and solid-phase polymerization reaction. The polymer obtained after solid phase polymerization was dissolved in o-chlorophenol with a concentration of 1.2 g/dl at 150°C for 15 minutes to measure its intrinsic viscosity (or solid phase intrinsic viscosity) at 35°C. The intrinsic viscosity (or solid phase intrinsic viscosity) may be 0.10 dl/g to 0.40 dl/g higher, or 0.15 dl/g to 0.25 dl higher than the intrinsic viscosity (or melting intrinsic viscosity) of the polymer after the polycondensation reaction. /g. Specifically, the intrinsic viscosity of the solid phase after the crystallization and solid phase polymerization reaction may be 0.6 dl/g to 1.0 dl/g, or 0.65 dl/g to 0.95 dl/g. When the intrinsic viscosity of the solid phase is within the above range, the molecular weight distribution of the polyester resin becomes narrow, thereby reducing the crystallization rate during the molding process. Therefore, heat resistance and crystallinity can be improved without reducing transparency.

In this disclosure, the intrinsic viscosity of the polyester resin can be measured by the time it takes for the solvent to pass through the specific internal area of the viscosity tube (efflux time; t0), and by dissolving the polyester resin. The time (t) it takes for the solution prepared in the solvent to pass through the tube is calculated using equations 2 and 3 below.

The polyester film according to an embodiment of the present disclosure is composed of polyethylene terephthalate (resin A) and polyester resin (resin B) in a weight ratio of 90:10 to 10:90 or 30:70 to 10:90. ) Is formed. Through the polyethylene terephthalate (resin A) and polyester resin (resin B) contained in the above mixing ratio, the light transmittance, heat resistance and adhesion of the resin film can be improved at the same time in a good balance degree. When the content of polyethylene terephthalate (resin A) is too high due to a mixing ratio exceeding 90:10, the improvement effect of the light transmittance and adhesion of the polyester resin (resin B) may not be significant. When the content of the polyester resin (resin B) is too high due to a mixing ratio exceeding 10:90, it may be difficult to use due to deterioration in heat resistance and increased strain.

The polyester film may be an unstretched film or a stretched film. When the polyester film is an unstretched film, it is preferable to mix polyethylene terephthalate (resin A) and polyester resin (resin B) in a weight ratio of 30:70 to 10:90 to improve light penetration Transmittance, while maintaining excellent heat resistance and adhesion.

In addition, when the polyester film is a stretched film, it may be a film stretched in at least one of the longitudinal and transverse directions, especially stretched at a total stretch ratio of 2 to 15 times or 4 to 12 times. film. When stretched at the above stretching ratio, heat resistance can be further improved.

More specifically, the polyester film may be a stretched film uniaxially stretched in any one of the longitudinal and transverse directions, and the stretch ratio in the longitudinal or transverse directions may be 2 to 15 times, 4 to 12 times or 5 to 10 times.

In addition, the polyester film may be a stretched film stretched in both directions in the longitudinal and transverse directions, and the longitudinal stretching ratio may be 2 to 5 times, 2 to 4 times, or 2 to 3 times, and the transverse stretching ratio may be 2 to 7. Times, 2 to 5 times, or 2 to 4 times. When stretched at the above stretching ratio, heat resistance can be further improved. In addition, when the polyester film is a stretched film stretched in both the longitudinal and transverse directions, the longitudinal stretch ratio and the transverse stretch ratio may be the same or different. In addition, in the case of biaxially stretched films with different stretching ratios, the stretching ratio in the longitudinal direction may be smaller than the stretching ratio in the transverse direction, and the stretching ratio in the longitudinal direction and the transverse direction may respectively satisfy the above-mentioned stretching ratio ranges.

In addition, the polyester film according to an embodiment of the present disclosure may further include at least one additive selected from crystallizing agents, sunscreen agents, antistatic agents, and impact modifiers. In the group consisting of impact modifiers, antioxidants and particles. The method of adding additives is not particularly limited. For example, it can be added to the preparation of polyester resin, or it can be added by making a high-concentration master batch of additives and diluting and mixing.

For example, with respect to the total weight of polyethylene terephthalate (resin A) and polyester resin (resin B), the polyester film may further include 5 to 200 ppm or 100 to 200 ppm of crystallization aid. Examples of crystallization aids can include crystal nucleating agents (silica, talc, aluminum hydroxide, boron nitride, etc.), ultraviolet absorbers (benzotriazole) , Benzophenone (benzophenone), salicylate (salicylate), cyanoacrylate (cyanoacrylate), oxanilide (oxanilide), hindered amine light stabilizer (HALS), etc.), polyolefin Polyolefin-based resin (polyethylene, polypropylene, etc.), polyamide resin, etc., and any one or a mixture of two or more of the above can be used as a crystal aid Agent. The heat resistance can be further improved by further including the crystal promoter within the above range.

The thickness of the polyester film may be appropriately determined according to its use, and specifically may be 1 micrometer (μm) to 2 mm. In the case of an unstretched film, the thickness may be 500 μm to 1 mm, and in the case of a stretched film, the thickness may be 1 μm to 350 μm.

In this disclosure, the thickness of the polyester film can be measured using an optical microscope, and unless otherwise specified, it means the average thickness.

The preparation method of the polyester film will be described in detail below.

The polyester film according to an embodiment of the present disclosure may include (a) mixing polyethylene terephthalate (resin A) and polyester resin (resin B) in a weight ratio of 90:10 to 10:90, Then melt extrusion to prepare a polyester film including a resin layer formed of polyethylene terephthalate (resin A) and polyester resin (resin B), and optionally including (b) in the poly The ester resin (resin B) is prepared by biaxially stretching the polyester film in the longitudinal and transverse directions at a temperature above the glass transition temperature. Therefore, according to another embodiment of the present disclosure, a method for preparing the above-mentioned polyester film is provided.

In the preparation method of the polyester film according to the embodiment of the present disclosure, step (a) is used to prepare an unstretched polymer from a mixture of polyethylene terephthalate (resin A) and polyester resin (resin B) Ester film. The polyethylene terephthalate (resin A) and polyester resin (resin B) here are as described above.

The mixing of polyethylene terephthalate (resin A) and polyester resin (resin B) in step (a) can be carried out according to the traditional mixing process, the difference is that each compound is used to meet the above mixing ratio conditions .

The melt extrusion in step (a) can be performed at a temperature of 240°C to 310°C or 250°C to 300°C. When the temperature is less than 240°C, the polymer may not melt. When the temperature exceeds 310°C, it may be difficult to achieve the expected physical properties because the thermal decomposition of the polymer increases and the film may be damaged or broken during film stretching. Therefore, the melt extrusion process can be performed at a relatively low temperature in the above range, thereby minimizing the thermal decomposition of the polymer and maintaining the long-chain structure.

In addition, other additives can be optionally added to improve the physical properties and effects of the polyester film to be prepared. The other additives are as described above, and may be added when the polyethylene terephthalate (resin A) and the polyester resin (resin B) are mixed and extruded together.

Due to the melt extrusion process, a sheet-like melt extrudate can be prepared, which is an unstretched polyester film. The unstretched polyester film prepared in step (a) can optionally undergo a cooling process to a suitable temperature, and the cooling process can be performed according to a conventional method.

Since the unstretched polyester film prepared in step (a) is formed of a mixture containing polyethylene terephthalate (resin A) and polyester resin (resin B) in an optimal mixing weight ratio, in addition to high gloss In addition to transmittance and low haze, it can also exhibit excellent heat resistance and adhesion.

In addition, when the polyester film according to the embodiment of the present disclosure is a stretched film, the preparation method of the polyester film may further include (step (b)) stretching the unstretched film prepared in step (a) step.

The stretching process may be performed at a temperature higher than or equal to the glass transition temperature of the polyester resin (resin B), specifically, at a temperature of 80°C to 180°C or 90°C to 170°C.

In addition, the stretching process can be performed by biaxially stretching an unstretched polyester film in the longitudinal and transverse directions. Specifically, the unstretched polyester film may be biaxially stretched at a stretching ratio of 2 to 5 times in the longitudinal direction and at a stretching ratio of 2 to 7 times in the transverse direction. In addition, it can be stretched at a total stretching ratio of 5 to 7 times while satisfying the stretching ratios in the longitudinal and transverse directions. By stretching at the above-mentioned high stretching ratio, the heat resistance of the resin film to be prepared can be further improved.

In addition, the preparation method of the polyester film provided in the embodiments of the present disclosure may further include (step (c)), after step (b), heat setting the polyester film obtained in step (b).

The heat setting process in step (c) can be carried out according to the traditional heat setting method, but the temperature is 100°C to 220°C. By performing the heat setting process in the above temperature range, the strain can be reduced by increasing the crystallinity of the resin film to be prepared, and the mechanical strength characteristics can be improved.

Meanwhile, the polyester resin (resin B) used in step (a) is a polyester resin in which isosorbide is incorporated in the above-mentioned content.

In order to prepare the polyester resin (resin B), the preparation method of the polyester film may also include before step (a): permeating a diol including (a0-1) p-dicarboxylic acid or its derivative and isosorbide The step of preparing polyester resin (resin B) by performing an esterification reaction or a transesterification reaction; and (a0-2) performing a polycondensation reaction on the product obtained through the esterification or transesterification reaction to perform a polycondensation reaction at 150°C After being dissolved in o-chlorophenol at a concentration of 1.2 g/dl for 15 minutes, a polyester resin with an intrinsic viscosity of 0.45 dl/g to 0.65 dl/g was prepared at 35°C.

The polyester resin (resin B) can be prepared in a batch, semi-continuous or continuous manner, and the esterification reaction or transesterification reaction of (a0-1) and the polycondensation reaction of (a0-2) can be prepared in an inert atmosphere. Under.

In the preparation of the polyester resin (resin B), relative to the total amount of the diol portion of the diol derived from the prepared polyester resin being 100 mol%, isosorbide is used so that The first diol part has a content of 4 mol% to 18 mol%. However, since some isosorbide may volatilize or not react during the synthesis of polyester resin (resin B), isosorbide can be used relative to the total amount of dicarboxylic acid or its derivatives as 100 mol%. The content is from 35 mol to 35 mol or from 5 mol to 30 mol in order to introduce isosorbide in the above content into the polyester resin (resin B). When the content of isosorbide exceeds the above range, yellowing may occur and the crystallinity may be significantly reduced, which may be detrimental to the stretching and heat setting process. When the content is less than the above range, it may not exhibit sufficient heat resistance and adhesion, resulting in haze. However, by controlling the content of isosorbide within the above-mentioned range, a polyester film having excellent heat resistance, adhesion, and transparency can be provided.

In addition, when the polyester resin (resin B) further includes a second diol moiety derived from cyclohexane dimethanol, cyclohexane dimethanol can be added in such a content that the second diol moiety satisfies the conditions of Formula 1 above.

In addition, the content of the third diol part derived from the aliphatic diol introduced into the polyester resin (resin B) is not directly proportional to the content of the aliphatic diol used to prepare the polyester resin. However, relative to the total amount of dicarboxylic acid or its derivatives as 100 mol%, the aliphatic diol can be used in a content of 90 mol to 120 mol or 95 mol to 115 mol so as to be As far as the total amount of the diol portion from the diol constituting the polyester resin is 100 mol%, the polyester resin includes 82 mol% to 96 mol% of the third diol portion derived from the aliphatic diol.

As mentioned above, in addition to the first diol part derived from isosorbide, the second diol part derived from cyclohexanedimethanol, and the third diol part derived from aliphatic diols, polyester resins (Resin B) may also include a fourth diol moiety derived from alicyclic diol in the residual content. The content of the cycloaliphatic diol added is such that relative to the total amount of the diol portion as 100 mol%, the fourth diol portion derived from the alicyclic diol is 0 mol% to 10 mol%, or 0.1 mol% % To 5 mol%.

In the esterification or transesterification reaction of (a0-1) used to prepare the polyester resin (resin B), the dicarboxylic acid or its derivative and the diol are in a stoichiometric ratio, and the molar ratio is 1:1, To react. However, dicarboxylic acid or its derivative and diol can be such that the molar ratio of diol to 1 mol of dicarboxylic acid or its derivative (molar ratio of diol/dicarboxylic acid or its derivative) is 1.01 Or greater content is added to the reactor.

For example, when dicarboxylic acid is used as dicarboxylic acid or its derivative, the initial mixing molar ratio of diol to dicarboxylic acid derivative can be adjusted to 1:1.01 to 1:1.5 or 1:1.05 to 1. :1.3. When using, for example, a derivative of dicarboxylic acid alkyl ester or dicarboxylic anhydride as the dicarboxylic acid or its derivative, the initial mixing of the diol and the dicarboxylic acid derivative The ratio can be adjusted from 1:2.0 to 1:2.5, or 1:2.1 to 1:2.3.

In addition, the diol may be added to the reactor at one time before the polymerization reaction, or added several times during the polymerization reaction. According to a more specific embodiment, a polyester resin satisfying a specific molecular weight distribution can be prepared by adjusting the initial content of the dicarboxylic acid or its derivative and diol to a specific range in the initial stage of the reaction. Therefore, the polyester film of the examples and the polyester resin contained therein can be provided more effectively. The initial mixing molar ratio may refer to the mixing molar ratio at the beginning of the polymerization reaction in the reactor, and if necessary, dicarboxylic acid or its derivatives and/or diols may be further added during the reaction.

In addition, the esterification or transesterification reaction of (a0-1) for preparing the polyester resin (resin B) can be carried out in a batch, semi-continuous or continuous manner. Each raw material can be added separately, but is preferably added in the form of a slurry in which a dicarboxylic acid or a derivative thereof is mixed with a glycol.

Further, the catalyst can be used for the esterification or transesterification reaction of (a0-1). Such catalysts may include: methylates of sodium and magnesium; zinc (Zn), cadmium (Cd), manganese (Mn), cobalt (Co), calcium (Ca), barium (Ba), titanium (Ti) Such as acetates, borates, fatty acids, carbonates or alkoxy salts; metals such as magnesium (Mg); and lead (Pb), zinc ( Zn), antimony (Sb), germanium (Ge), etc., and preferably germanium oxide (GeO2), antimony oxide (Sb2O3), or antimony oxide and manganese (Mn(b)) acetate tetrahydrate (acetate tetrahydrate) )mixture. The catalyst can be used in a molar ratio of 1 to 3 or 1.05 to 2.5 with respect to 1 mol of dicarboxylic acid or a derivative thereof.

In addition, at least one of polycondensation catalysts, stabilizers, colorants, crystal promoters, antioxidants, and branching agents may be further added to the slurry or in the reaction before the esterification or transesterification reaction of (a0-1). After completion, it is further added to the product. However, the present disclosure is not limited to this, and the above-mentioned additives can be added at any time during the preparation process of the polyester resin.

At least one of conventional titanium, germanium, antimony, aluminum, and tin-based compounds can be appropriately selected and used as a polycondensation catalyst. Examples of preferred titanium-based catalysts include tetraethyl titanate, acetyltripropyl titanate, tetrapropyl titanate, and tetrapropyl titanate. Butyl (tetrabutyl titanate), polybutyl titanate (polybutyl titanate), 2-ethylhexyl titanate (2-ethylhexyl titanate), octylene glycol titanate (octylene glycol titanate), titanate (lactate titanate) , Triethanolamine titanate, acetylacetonate titanate, ethyl acetoacetic ester titanate, isostearyl titanate, titanium dioxide (titanium dioxide), titanium dioxide/silicon dioxide copolymer (titanium dioxide/silicon dioxide copolymer), titanium dioxide/zirconium dioxide copolymer (titanium dioxide/zirconium dioxide copolymer), etc. In addition, examples of preferred germanium-based catalysts include germanium dioxide and its copolymers. With respect to the total weight of the final polymer (polyester resin) based on the central metal atom, the addition amount of the polycondensation catalyst may be 1-300 ppm.

As a stabilizer, phosphorus-based compounds such as phosphoric acid, trimethyl phosphate, and triethyl phosphate can generally be used, compared to the final polymer based on phosphorus atoms (polyester resin) In terms of weight, the added content of phosphorus-based compounds can be 10 to 5000 ppm. When the content of the stabilizer is less than 10 ppm, the polyester resin may not be stable enough, and the color of the polyester resin may become yellow. When the content is more than 5000 ppm, a polymer with a high degree of polymerization cannot be obtained.

Further, examples of coloring agents added to improve the color of the polymer may include traditional cobalt-based coloring agents, such as cobalt acetate, cobalt propionate, and the like. The content of the colorant added may be 1 to 300 ppm relative to the weight of the final polymer (polyester resin) based on cobalt atoms. If necessary, anthraquinone-based compounds, perinone-based compounds, azo-based compounds, methine-based compounds, etc. can be used as Organic colorants, and commercially available products may include toners, such as Polysynthrene Blue RLS manufactured by Clarient Corporation or Solvaperm Red BB manufactured by Clarient Corporation. Relative to the total weight of the final polymer (polyester resin), the addition amount of the coloring agent of the organic compound may be 0 ppm to 50 ppm. When the colorant is used in a content outside the above range, the yellow color of the polyester resin may not be sufficiently hidden or the physical properties may be deteriorated.

Examples of crystallization aids may include crystal nucleating agents, ultraviolet absorbers, polyolefin-based resins, polyamide resins, etc., and relative to the weight of the final polymer (polyester resin), the amount of crystallization aids added may be It is 1 to 200 ppm.

Examples of antioxidants may include hindered phenolic antioxidants, phosphite-based antioxidants, thioester-based antioxidants, and mixtures thereof. Relative to the total weight of the final polymer (polyester resin), the amount of antioxidant added can be 10 to 500 ppm.

Examples of branching agents may include traditional branching agents having three or more functional groups, such as trimellitic anhydride, trimethylol propane, trimellitic acid, and mixtures thereof. Relative to the total weight of the final polymer (polyester resin), the added content of the branching agent can be 10 to 300 ppm.

In addition, the esterification or transesterification reaction of (a0-1) used to prepare the polyester resin (resin B) can be performed at a temperature of 150°C to 300°C or 200°C to 270°C and a pressure of 0 to 10.0 kgf/ Cm2 (kgf/cm ² ) (0 to 7355.6 mmHg (mmHg)), 0 to 5.0 kgf/cm ² (0 to 3677.8mmHg) or 0.1 to 3.0 kgf/cm ² (73.6 to 2206.7mmHg) get on. The pressure outside the brackets refers to the gauge pressure (expressed in kgf/cm ² ), and the pressure in the brackets refers to the absolute pressure (expressed in mmHg). When the reaction temperature and pressure exceed the above ranges, the physical properties of the polyester resin may be deteriorated. The reaction time (average residence time) is usually 1 hour to 24 hours or 2 hours to 8 hours, which can depend on the reaction temperature, pressure and the molar ratio of the diol to the dicarboxylic acid or its derivative. The ear ratio changes.

The product obtained by the esterification or transesterification reaction of (a0-1) can prepare a polyester resin with a high degree of polymerization through the following polycondensation reaction (a0-2).

The polycondensation reaction of (a0-2) can be performed at a temperature of 150°C to 300°C, 200°C to 290°C, or 250°C to 290°C under reduced pressure of 0.01 to 400 mmHg, 0.05 to 100 mmHg, or 0.1 to 10 mmHg get on. The pressure here refers to absolute pressure. The reduced pressure conditions of 0.01 to 400 mmHg are used to remove by-products of the polycondensation reaction, such as glycol, and unreacted substances, such as isosorbide. Therefore, when the pressure exceeds the above range, by-products and unreacted substances may not be sufficiently removed. In addition, when the reaction temperature of the polycondensation reaction exceeds the above-mentioned range, the physical properties of the polyester resin may be deteriorated. The polycondensation reaction can be carried out for a period of time until the desired intrinsic viscosity is reached. For example, the average residence time carried out can be 1 hour to 24 hours.

In order to reduce the content of unreacted substances such as isosorbide remaining in the polyester resin, the vacuum reaction can be intentionally maintained for a period of time in the final stage of the esterification reaction or transesterification reaction or in the initial stage of the polycondensation reaction. In other words, the unreacted raw materials can be discharged from the system when the resin viscosity is not high enough. When the resin viscosity is high, the raw materials remaining in the reactor cannot easily flow out of the system. For example, before the polycondensation reaction, the reaction product obtained by the esterification reaction or the transesterification reaction is placed under a reduced pressure of about 400 to 1 mmHg or about 200 to 3 mmHg for 0.2 hours to 3 hours. Effectively remove unreacted substances such as isosorbide remaining in the polyester resin. The temperature of the product can be controlled to be equal to the temperature of the esterification or transesterification reaction or the temperature of the polycondensation reaction, or a temperature between the esterification or transesterification reaction and the polycondensation reaction.

By controlling the vacuum reaction and adding a process for flowing unreacted substances out of the system, the content of unreacted substances such as isosorbide remaining in the polyester resin can be reduced. Therefore, the polyester film and the polyester resin contained therein that satisfy the physical properties of the examples can be obtained more effectively.

At the same time, the melt intrinsic viscosity of the polymer obtained after the polycondensation reaction of (a0-2) is preferably 0.45 dl/g to 0.65 dl/g. When the intrinsic viscosity is less than 0.45 dl/g, the reaction rate of solid-phase polymerization may be significantly reduced. When the intrinsic viscosity exceeds 0.65 dl/g, the viscosity of the molten material may increase during melt polymerization, resulting in the probability of polymer discoloration due to the shear stress between the stirrer and the reactor (shear stress) And increase, and produce by-products such as acetaldehyde.

The melting intrinsic viscosity of the polymer here is measured at 35°C after dissolving the polymer in o-chlorophenol at a concentration of 1.2 g/dl at 150°C for 15 minutes.

Through the above steps (a0-1) and (a0-2), the polyester resin used to form the polyester film of an embodiment of the present disclosure can be prepared.

After the polycondensation reaction is completed, the obtained polymer can be further subjected to solid-phase polymerization after crystallization to prepare a polyester resin having a uniform molecular weight distribution, so the transparency of the prepared film can be further improved.

Thereby, the preparation method of the polyester film of an embodiment of the present disclosure may include after the polycondensation reaction step of (a0-2): (a0-3) the polyester resin obtained by the polycondensation reaction (melt polymerization) ( (Hereinafter referred to as polymer) undergoing a crystallization process; and (a0-4) performing a solid-phase polymerization process on the crystallized polymer to have an intrinsic viscosity, which is dissolved in a concentration of 1.2 g/dl at 150°C O-chlorophenol is measured at 35°C after 15 minutes, and is 0.10 to 0.40 dl/g higher than the intrinsic viscosity of the polymer obtained in step (a0-2).

Specifically, in the crystallization step of (a0-3), the polymer obtained by the polycondensation reaction of (a0-2) can be discharged out of the reactor to be granulated. The method of making pellets can be the strand cutting method in which the polymer is extruded into a strand shape and solidified in the cooling liquid and then cut with a cutting machine, or the die hole is immersed in the cooling liquid The underwater cutting method in which the polymer is directly extruded in the cooling liquid and then cut with a cutting machine (underwater cutting method). In the strand cutting method, the coolant is generally maintained at a low temperature to fully solidify the strands, thereby avoiding cutting problems. In the underwater cutting method, it is preferable to maintain the temperature of the cooling liquid to conform to the polymer, so that the shape of the polymer is consistent. However, in the case of crystalline polymers, the temperature of the cooling liquid can be intentionally maintained at a high temperature, thereby generating crystallization during discharge (discharge).

At the same time, water can also be used to clean the polymer made into particles. During the cleaning, the temperature of the water is preferably the same as the glass transition temperature of the polymer or is about 5°C to 20°C lower than the glass transition temperature. When the temperature of the water is higher than the above range, it is not preferable because of the possibility of fusion. In the case where the polymer particles are crystallized during the discharge process, even if the temperature is higher than the glass transition temperature, fusion will not occur, so the temperature of the water can be determined according to the crystallinity. By washing the particulate polymer with water, the raw materials dissolved in water in the unreacted raw materials can be removed. As the particle size is smaller, the surface area of the particle under a certain weight will be larger, so a smaller particle size is more advantageous. In order to achieve such a purpose, the particles may be prepared to have an average weight of about 14 milligrams (mg) or less.

The pelletized polymer will first undergo a crystallization step to prevent fusion during the solid phase polymerization step. The crystallization step can be carried out at a temperature of 110°C to 180°C or 120°C to 180°C under the atmosphere, inert gas, water vapor, inert gas containing water vapor, or in a solution. When the temperature is low, the speed of forming granular crystals may be too slow. When the temperature is high, the surface of the particles will melt faster than the rate at which crystals are formed, causing the particles to stick together, leading to fusion. Since the heat resistance of the particles increases as the particles crystallize, the particles can also be crystallized by dividing the crystallization process into several steps and gradually increasing the temperature.

The solid-phase polymerization reaction can be under an inert gas environment such as nitrogen, carbon dioxide, or argon, or under a reduced pressure of 400 to 0.01 mmHg at a temperature of 180 to 220°C The average residence time is continued for 1 hour or more, preferably 10 hours or more. Through this solid-phase polymerization reaction, the molecular weight can be further increased, and the unreacted raw materials remaining in the melt reaction and the cyclic oligomers and acetaldehyde produced during the reaction can be removed.

The solid phase polymerization reaction can be carried out until the solid phase intrinsic viscosity of the polymer is 0.10 dl/g to 0.40 dl/g higher than the melting intrinsic viscosity of the polymer obtained by the polycondensation reaction of (a0-2). When the difference between the intrinsic viscosity of the resin after the solid-phase polymerization and the intrinsic viscosity of the resin before the solid-phase polymerization is less than 0.10 dl/g, the degree of polymerization may not be effectively improved. When the difference in intrinsic viscosity exceeds 0.40 dl/g, the molecular weight distribution becomes broad, so that sufficient heat resistance cannot be exhibited. In addition, as the content of oligomers relatively increases, crystallization is likely to occur at high temperatures, making it difficult to maintain high transparency after heat treatment.

More specifically, the solid-phase polymerization reaction can be carried out until the solid-phase intrinsic viscosity of the polymer is 0.10 dl/g to 0.40 dl/g higher than the melting intrinsic viscosity of the polymer before the solid-phase polymerization reaction, and the value is 0.65 dl/g to 1.5 dl/g, 0.7 dl/g to 1.2 dl/g, or 0.8 dl/g to 1.0 dl/g. When the solid-phase polymerization is carried out until the intrinsic viscosity reaches the above range, the molecular weight distribution of the polymer can be narrowed, thereby reducing the crystallization rate during the molding process. Therefore, heat resistance and crystallinity can be improved without reducing transparency. When the intrinsic viscosity of the resin after solid-phase polymerization is less than the above range, since the crystallization rate is increased due to the low molecular weight polymer, it is difficult to provide a polyester film with excellent transparency.

The polyester resin (resin B) prepared according to the above method has a repeating structure of an acid part derived from a dicarboxylic acid or its derivative and a diol part derived from a diol, and is relative to the total amount of the diol part For 100 mol%, it contains 4 mol% to 18 mol% of the first diol moiety derived from isosorbide and optionally contains the second diol moiety derived from cyclohexanedimethanol. When the polyester resin further includes a second diol moiety derived from cyclohexanedimethanol, the content of the second diol moiety meets the conditions of Formula 1 above. Therefore, when the polyester resin is applied to the preparation of the film, it can exhibit good heat resistance and good adhesion.

The polyester film of an embodiment of the present disclosure prepared by the above-mentioned preparation method may be a single-layer film composed of a resin layer formed of a mixture of polyethylene terephthalate and polyester resin in an optimal mixing ratio.

Therefore, the polyester film can exhibit high transparency, and in particular, when the polyester film is an unstretched film with a thickness of 1 mm or a stretched film with a thickness of 200 μm, the haze measured according to ASTM D1003-97 can be It is 3% or less, 2% or less, 1.5% or less, 1% or less, or 0.5% or less. The lower limit of the haze is not particularly limited, and may be 0%.

In addition, the polyester film may exhibit good heat resistance, and specifically, in the case of an unstretched film, the loss factor (tan δ, tan delta) may be 88°C or higher, or 88°C to 100°C. Therefore, the polyester film can be easily used in a printing process at a temperature of about 80°C. In the case of a stretched film, the tan δ can be 110°C or higher, or 110°C to 130°C, so it can be used in a printing process at a higher temperature.

Heat resistance can usually be evaluated by the glass transition temperature measured by differential scanning calorimetry. However, if differential scanning calorimetry cannot be used due to the nature of the sample, dynamic mechanical analysis (DMA) or thermomechanical analysis (TMA) can be used to measure Young’s modulus E'and loss modulus. Calculate tan δ from them to obtain the glass transition temperature. Therefore, in this disclosure, tan δ is calculated by dynamic mechanical analysis, and the heat resistance is evaluated based on this result. Specifically, the poly The ester film was cut into a size of 30 millimeters (mm) x 5.3 millimeters (longitudinal length x transverse length) to prepare samples, and the Young's modulus E'and loss modulus E" were measured using dynamic mechanical analysis under the following conditions. Then the tan δ can be calculated according to Equation 4 below.

>Measurement conditions>

Fixed frequency (scanning frequency/amplitude: 15 μm))

Temperature change: The temperature increases from room temperature (RT) to 150°C at a rate of 3°C per minute

[Formula 4]

Tan δ= E’/E”

If tan δ is 100°C or higher, or more specifically 110°C or higher, it can be judged as "excellent heat resistance."

In addition, the polyester film may have low strain due to excellent heat resistance. Specifically, when the polyester film is an unstretched film with a thickness of 1 mm or a stretched film with a thickness of 200 μm, its strain at 100° C. may be 3% or less, or 1% to 3%.

In this disclosure, the strain (%) can be calculated by the Creep TTS test using dynamic mechanical analysis (DMA) and time-temperature superposition (TTS). Specifically, in the creep TTS test, the polyester film will heat up from room temperature, and when the temperature reaches 100° C., a stress of 10 megapascals (MPa) is applied to the film under isothermal conditions for 10 minutes. Then, according to the following formula 5, relative to the film length before the stress is applied, the deformation of the film length when the stress is applied at 100° C. is converted into strain (%).

[Formula 5]

Strain (%)=[(length of polyester film after applying stress at 100°C-length of polyester film before applying stress)/length of polyester film before applying stress]×100

When the strain at 100°C exceeds 3%, the degree of deformation can be visually recognized. Therefore, a strain of 3% or less at 100°C can be evaluated as "excellent heat resistance".

In addition, the polyester film may have excellent adhesion, and in particular, may have good adhesion to paper.

As described above, the polyester film according to an embodiment of the present disclosure can be applied in various fields due to its excellent heat resistance, transparency and adhesion. In particular, polyester films can be used for optical films that require high transparency, food container films or printing films that require excellent heat resistance and chemical resistance, and adhesive films or packaging films that require high adhesiveness and heat sealability.

Hereinafter, the steps and effects of the present invention will be described in detail through specific examples of the present invention. However, the examples are presented as examples and are not intended to limit the scope of the present invention.

The following physical properties were measured according to the following methods.

(1) Intrinsic viscosity (IV)

[公式3]

After dissolving the sample in o-chlorophenol with a concentration of 1.2 g/dl at 150°C for 15 minutes, the intrinsic viscosity of the sample was measured using an Ubbelohde viscosity tube. Specifically, the temperature of the viscosity tube is maintained at 35°C, and the time (outflow time; t0) spent between the solvent (o-chlorophenol) passing through a specific internal area of the viscosity tube is measured, and the sample is prepared by dissolving the sample in the solvent. The time (t) taken for the solution to pass through the tube. Subsequently, substituting t0 and t into Equation 2 to calculate the specific viscosity η _sp , and substituting the calculated specific viscosity into Equation 3 to calculate the intrinsic viscosity. [Formula 2]

[Formula 3]

In Equation 3, A is the Huggin's constant of 0.247, and c is the concentration of 1.2 g/dl.

In the case of melting intrinsic viscosity, the polymer system obtained after the polycondensation reaction is used as the sample, and in the case of solid phase intrinsic viscosity, the polymer obtained after crystallization and solid-phase polymerization is used as the sample.

(2) The content of the diol moiety derived from isosorbide (ISB) and 1,4-cyclohexanedimethanol (CHDM)

After the final prepared polyester resin sample is dissolved in a deuterated chloroform (CDCI ₃ ) solvent with a concentration of 3 milligrams/ml (mg/mL), it is tested by using a nuclear magnetic resonance device (JEOL, 600 at 25°C). MHz FT-NMR), the 1H-NMR spectrum obtained can confirm the content of the first diol part derived from isosorbide (ISB) and the second second part derived from 1,4-cyclohexanedimethanol (CHDM) The content of the alcohol part.

(3) Film thickness

The cross section of the polyester film prepared in one of the Examples and Comparative Examples was observed with an optical microscope. After determining the thickness of different positions, the average value can be judged as the film thickness.

(4)tan δ

In order to evaluate the heat resistance of the polyester film, the polyester film prepared in one of the Examples and Comparative Examples was cut into a size of 30 mm × 5.3 mm (longitudinal length × lateral length) to prepare samples, and under the following conditions, respectively Use DMA to measure Young's modulus (E') and loss modulus (E"). After that, tanδ is calculated according to the following formula 4.

>Measurement conditions>

Fixed frequency (scanning frequency/amplitude: 15μm))

Temperature change: The temperature increases from room temperature (RT) to 150°C at a rate of 3°C per minute

[Formula 4]

Tan δ= E’/E”

If tan δ is 100°C or higher, or more specifically 110°C or higher, it can be judged as "excellent heat resistance".

(5) Strain

When stress is applied to the sample, the sample will deform corresponding to the stress. Even if the sample is subjected to a fixed stress, the creep phenomenon of the sample gradually deforming over time will occur.

Therefore, in order to predict the deformation based on the temperature of the polyester film prepared from one of the Examples and Comparative Examples, a dynamic mechanical analysis (DMA) and time-temperature superposition (TTS) can be used to perform a creep TTS test.

Specifically, in the creep TTS test, a sample having a size of 30 mm × 5.3 mm (longitudinal length × transverse length) of a polyester film prepared by one of the Examples and Comparative Examples was heated from room temperature to room temperature ( RT), and when the temperature reaches 90°C, 100°C, and 110°C, a stress of 10 MPa is applied to the sample for 10 minutes under isothermal conditions, respectively. After that, according to Equation 6 below, relative to the length of the sample before the stress is applied, the deformation of the sample length when stress is applied at 90°C, 100°C, or 110°C is converted into strain (%).

[Formula 6]

Strain (%)=[(length of the polyester film sample after stress is applied at 90℃, 100℃ or 110℃-the length of the polyester film sample before stress is applied)/the polyester film sample before stress is applied Length)]×100

For example, the strain at 100°C can be obtained by substituting the length of a sample of a polyester film deformed by applying a stress of 10 MPa for 10 minutes under an isothermal condition of 100°C to the polyester film after stress at 100°C The length of the sample (see Equation 5 above) is calculated.

According to the results of confirming the deformation at the temperatures of 90°C, 100°C, and 110°C, the strain at 90°C is smaller than the strain at 100°C, and the strain at 110°C is higher than the strain at 100°C. Therefore, the heat resistance can be evaluated based on the strain value at 100°C. When the strain at 100°C exceeds 3%, the degree of deformation can be visually recognized. Therefore, a 100°C strain of 3% or less can be evaluated as "excellent heat resistance".

(6) Adhesion

Using a thermal gradient tester, the polyester film prepared in one of the example and the comparative example was placed between the paper, and then bonded at 230°C for 3 seconds to confirm and evaluate the adhesion to the paper according to the following standards .

O: Bonded

X: Not bonded

(7) Haze

The polyester film prepared in one of the Examples and Comparative Examples was cut into a size of 10 cm×10 cm (longitudinal length×horizontal length) to prepare samples. The parallel penetration rate and diffusion penetration rate of the sample can be measured according to ASTM D1003-97 using the CM-3600A instrument manufactured by Minolta. The penetration rate is defined as the sum of the parallel penetration rate and the diffusion penetration rate, and the haze is defined as the percentage of the diffusion penetration rate to the penetration rate (haze = diffusion penetration rate / penetration rate × 100). Therefore, the haze can be calculated from the parallel penetration rate and diffusion penetration rate of the sample.

The haze of 3 or less can be evaluated as "excellent transparency".

>Preparation of polyester resin>

Preparation example 1

Put 3257.4 g (19.6 mol (mol)) terephthalic acid (TPA), 1180.1 g (19.0 mol) ethylene glycol, and 229.2 g (1.6 mol) isosorbide (ISB) into a volume of 10 liters (L ) In the reactor, in which a water-cooled column and condenser are connected to the reactor, but relative to the total amount of the diol part, the content can be adjusted so as to be incorporated into the final polyester resin The content of the diol moiety derived from isosorbide is 5 mol%. 1.0 g of germanium dioxide (the molar ratio of germanium dioxide to terephthalic acid is 1.05) was used as a catalyst, 1.46 g of phosphoric acid was used as a stabilizer, and 0.7 g of cobalt acetate was used as a colorant. Next, nitrogen was injected into the reactor to form a pressurized state, in which the pressure of the reactor was 1.0 kgf/cm ² higher than the normal pressure.

In addition, the temperature of the reactor was increased to 260°C and maintained at the same temperature, and the esterification reaction was performed until the mixture in the reactor became transparent. In this process, unreacted isosorbide and by-products can be discharged through the column and condenser. When the esterification reaction is completed, the nitrogen in the reactor in a pressurized state is discharged to the outside to reduce the pressure in the reactor to normal pressure, and then the mixture in the reactor is transferred to a volume capable of performing a vacuum reaction. In a liter reactor.

Next, the temperature of the reactor was increased to 280° C. over 1 hour, and the pressure of the reactor was maintained at a pressure of 1 Torr or less while performing the polycondensation reaction. The polycondensation reaction continues until the intrinsic viscosity (melting intrinsic viscosity) of the reaction product in the reactor becomes 0.55 dl/g. When the intrinsic viscosity of the reaction product reaches the desired level, the reaction product is discharged to the outside of the reactor and twisted, then solidified with a cooling liquid and made into pellets, so that the average weight is about 12 to 14 mg.

The particles were placed in a nitrogen atmosphere at 150°C for 1 hour and crystallized, and then added to a solid phase polymerization reactor with a volume of 20 liters. After that, nitrogen gas was flowed into the reactor at a rate of 50 liters per minute (L/min). Here, the temperature of the reactor was increased from room temperature to 140°C at a rate of 40°C/hour (°C/hour), and maintained at 140°C for 3 hours. Then, the temperature was increased to 200°C again at a rate of 40°C/hour and maintained at 200°C. The solid phase polymerization reaction is carried out until the intrinsic viscosity of the particles in the reactor (solid phase intrinsic viscosity) reaches 0.70 dl/g.

The content of the acid moiety derived from terephthalic acid is 100 mol% relative to the total amount of the acid moiety contained in the obtained polyester resin. The content of the diol part derived from isosorbide is 5 mol% with respect to the total amount of the diol part.

Preparation example 2

Put 3354.8 g (20.2 mol) of terephthalic acid (TPA), 1403.4 g (22.6 mol) of ethylene glycol and 531.1 g (3.6 mol) of isosorbide (ISB) into a 10 L reactor, A column and a condenser that can be cooled with water are connected to the reactor, but the content can be adjusted so that the content of the ISB-derived diol part introduced into the final polyester resin is 10 mol% relative to the total amount of the diol part . 1.0 g of germanium dioxide (germanium dioxide/terephthalic acid molar ratio of 1.3) was used as a catalyst, 1.46 g of phosphoric acid was used as a stabilizer, and 0.7 g of cobalt acetate was used as a colorant. Then, nitrogen was injected into the reactor to form a pressurized state, in which the pressure of the reactor was 1.0 kgf/cm ² higher than the normal pressure.

In addition, the temperature of the reactor was increased to 260°C and maintained at the same temperature, and the esterification reaction was performed until the mixture in the reactor became transparent. In this process, unreacted ISB and by-products flow out through the tube and condenser. When the esterification reaction is completed, the nitrogen in the pressurized reactor is discharged to the outside to reduce the pressure of the reactor to normal pressure, and then the mixture in the reactor is transferred to a 7 L reactor capable of vacuum reaction .

Then, the temperature of the reactor was raised to 270°C within 1 hour, and the polycondensation reaction continued while maintaining the pressure of the reactor at 1 Torr or lower. The polycondensation reaction proceeds until the intrinsic viscosity (melting IV) of the reaction product in the reactor becomes 0.50 dl/g. When the intrinsic viscosity of the reaction product reaches the expected level, the reaction product is discharged from the reactor and twisted. It is solidified with coolant and made into pellets so that the average weight is about 12 mg to 14 mg.

The particles were stored in water at 70°C for 5 hours, and allowed to stand at 150°C for 1 hour under a nitrogen atmosphere for crystallization, and then put into a 20 L solid phase polymerization reactor. Then, nitrogen gas flows into the reactor at a rate of 50 L/min. Here, the temperature of the reactor was increased from room temperature to 140°C at a rate of 40°C/hour and maintained at 140°C for 3 hours. Then, the temperature was further increased to 200°C at a rate of 40°C/hour and kept at 200°C. The solid phase polymerization reaction is carried out until the intrinsic viscosity of the particles in the reactor (solid phase IV) reaches 0.75 dl/g.

Preparation example 3

3110.9 g (18.7 mol) of terephthalic acid, 1161.9 g (18.7 mol) of ethylene glycol and 820.8 g (5.6 mol) of isosorbide (ISB) are placed in a 10 L reactor, which can be cooled by water The column and condenser are connected to the reactor, but the content can be adjusted so that the content of the ISB-derived diol part introduced into the final polyester resin is 16 mol% relative to the total amount of the diol part. 1.0 g of germanium dioxide (germanium dioxide/terephthalic acid molar ratio of 1.3) was used as a catalyst, 1.46 g of phosphoric acid was used as a stabilizer, and 0.015 g of Polysynthrene Blue RLS (manufactured by Clarient) was used as a blue tint Toner, 0.004 g of Solvaperm Red BB (manufactured by Clarient) was used as a red toner, 1 ppm of polyethylene was used as a crystallization aid, 100 ppm of antioxidant (Iganox 1076) and 100 ppm of trimellitic anhydride (trimellitic anhydride) were used. anhydrate) as a branching agent. Then, nitrogen was injected into the reactor to form a pressurized state, in which the pressure of the reactor was 1.0 kgf/cm ² higher than the normal pressure.

In addition, the temperature of the reactor was increased to 260°C and maintained at the same temperature, and the esterification reaction was carried out until the mixture in the reactor became transparent. During this process, unreacted ISB and by-products flow out through the column and condenser. After the esterification reaction is completed, the nitrogen in the pressurized reactor is discharged to the outside to reduce the pressure of the reactor to normal pressure, and then the mixture in the reactor is transferred to a 7 L reactor capable of vacuum reaction.

Then, the temperature of the reactor was increased to 275°C within 1 hour, and the polycondensation reaction continued while maintaining the pressure of the reactor at 1 Torr or lower. The polycondensation reaction proceeds until the intrinsic viscosity (melting IV) of the reaction product in the reactor becomes 0.60 dl/g. When the intrinsic viscosity of the reaction product reaches the expected level, the reaction product is discharged from the reactor and twisted. It is solidified with coolant and made into pellets so that the average weight is about 12 mg to 14 mg.

Preparation example 4

3775.4 g (19.5 mol) of dimethyl terephthalate (DMT), 2654.5 g (42.8 mol) of ethylene glycol and 852.4 g (5.8 mol) of isosorbide (ISB) were placed in a 10 L reactor , The column and condenser that can be cooled with water are connected to the reactor, but the content can be adjusted so that the content of the ISB-derived diol part introduced into the final polyester resin relative to the total amount of the diol part is 10 mol%. Take 1.5 g of manganese (II) acetate tetrahydrate (Mn(II) acetate tetrahydrate) and 1.8 g of antimony trioxide (Sb ₂ O ₃ ) ((manganese (II) acetate tetrahydrate + antimony trioxide)/DMT The molar ratio of 2.5) was used as a catalyst, 1.46 g phosphoric acid was used as a stabilizer, and 0.7 g cobalt acetate was used as a colorant. Then, nitrogen was injected into the reactor to form a pressurized state, in which the pressure of the reactor was 1.0 kgf/cm ² higher than the normal pressure.

In addition, the temperature of the reactor was increased to 240°C and kept at the same temperature, and the esterification reaction was carried out until the mixture in the reactor became transparent. During this process, unreacted ISB and by-products flow out through the column and condenser. After the esterification reaction is completed, the nitrogen in the pressurized reactor is discharged to the outside to reduce the pressure of the reactor to normal pressure, and then the mixture in the reactor is transferred to a 7 L reactor capable of vacuum reaction .

Then, the temperature of the reactor was raised to 265° C. within 1 hour, and the polycondensation reaction was performed while maintaining the pressure of the reactor at 1 Torr or lower. The polycondensation reaction proceeds until the intrinsic viscosity (melting IV) of the reaction product in the reactor becomes 0.50 dl/g. When the intrinsic viscosity of the reaction product reaches the expected level, the reaction product is discharged from the reactor and twisted. It is solidified with coolant and made into pellets so that the average weight is about 12 mg to 14 mg.

The particles were allowed to stand at 150°C for 1 hour under a nitrogen atmosphere to crystallize, and then put into a 20 L solid phase polymerization reactor. Then, nitrogen gas flows into the reactor at a rate of 50 L/min. Here, the temperature of the reactor was increased from room temperature to 140°C at a rate of 40°C/hour, and maintained at 140°C for 3 hours. Thereafter, the temperature was further increased to 200°C at a rate of 40°C/hour and kept at 200°C. The solid phase polymerization reaction is carried out until the intrinsic viscosity of the particles in the reactor (solid phase IV) reaches 0.75 dl/g.

Preparation Example 5

3226.4 g (19.4 mol) of terephthalic acid (TPA), 169.8 g (1.0 mol) of isophthalic acid (IPA), 1420.7 g (22.9 mol) of ethylene glycol, and 537.7 g (3.7 mol) Isosorbide (ISB) is placed in a 10 L reactor, and a pipe column that can be cooled by water and a condenser are connected to the reactor, but the content can be adjusted so that relative to the total amount of the glycol part, it is introduced into the final polymer The content of the diol portion derived from ISB of the ester resin is 10 mol%. 1.0 g of germanium dioxide (the total molar ratio of germanium dioxide to terephthalic acid and isophthalic acid is 1.3) as a catalyst, 1.46 g of phosphoric acid as a stabilizer, and 0.7 g of cobalt acetate As a colorant. Next, nitrogen was injected into the reactor to form a pressurized state, in which the pressure of the reactor was 1.0 kgf/cm ² higher than the normal pressure.

In addition, the temperature of the reactor was increased to 260°C and kept at the same temperature, and the esterification reaction continued until the mixture in the reactor became transparent. During this process, unreacted ISB and by-products flow out through the column and condenser. After the esterification reaction is completed, the nitrogen in the pressurized reactor is discharged to the outside to reduce the pressure of the reactor to normal pressure, and then the mixture in the reactor is transferred to a 7 L reactor capable of vacuum reaction.

Then, the temperature of the reactor was increased to 280°C within 1 hour, and the polycondensation reaction continued while maintaining the pressure of the reactor at 1 Torr or lower. The polycondensation reaction proceeds until the intrinsic viscosity (melting IV) of the reaction product in the reactor becomes 0.50 dl/g. When the intrinsic viscosity of the reaction product reaches the expected level, the reaction product is discharged from the reactor and twisted. It is solidified with coolant and made into pellets so that the average weight is about 12 mg to 14 mg.

The particles were allowed to stand at 150°C for 1 hour in a nitrogen atmosphere for crystallization, and then put into a 20 L solid phase polymerization reactor. Then, nitrogen gas flows into the reactor at a rate of 50 L/min. Here, the temperature of the reactor was increased from room temperature to 140°C at a rate of 40°C/hour, and maintained at 140°C for 3 hours. Thereafter, the temperature was further increased to 200°C at a rate of 40°C/hour and kept at 200°C. The solid phase polymerization reaction is carried out until the intrinsic viscosity of the particles in the reactor (solid phase IV) reaches 0.75 dl/g.

Preparation Example 6

3112.7 g (18.8 mol) of terephthalic acid (TPA), 1209.1 g (19.5 mol) of ethylene glycol (EG), 162.0 g (1.1 mol) of cyclohexane dimethanol (CHDM) and 547.4 g (3.8 mol) of isosorbide (ISB) was placed in a 10 L reactor, and a water-cooled column and a condenser were connected to the reactor, but the content could be adjusted so that relative to the total amount of diol It is said that the content of the diol part derived from ISB and the diol part derived from CHDM into the final polyester resin can be 10 mol% and 6 mol%, respectively. 1.0 g of germanium dioxide (the molar ratio of germanium dioxide to terephthalic acid is 1.05) is used as a catalyst, 1.46 g of phosphoric acid is used as a stabilizer, and 0.9 g of cobalt acetate is used as a colorant. Next, nitrogen was injected into the reactor to form a pressurized state, in which the pressure of the reactor was 1.0 kgf/cm ² higher than the normal pressure.

In addition, the temperature of the reactor was increased to 260°C and kept at the same temperature, and the esterification reaction was carried out until the mixture in the reactor became transparent. During this process, unreacted ISB and by-products flow out through the column and condenser. After the esterification reaction is completed, the nitrogen in the pressurized reactor is discharged to the outside to reduce the pressure of the reactor to normal pressure, and then the mixture in the reactor is transferred to a 7 L reactor capable of vacuum reaction.

Then, the temperature of the reactor was increased to 270° C. within 1 hour, and the polycondensation reaction was performed while maintaining the pressure of the reactor at 1 Torr or lower. The polycondensation reaction proceeds until the intrinsic viscosity (melting IV) of the reaction product in the reactor becomes 0.70 dl/g. When the intrinsic viscosity of the reaction product reaches the expected level, the reaction product is discharged from the reactor and twisted. It is solidified with coolant and made into pellets so that the average weight is about 12 mg to 14 mg.

Comparative preparation example 1

3456.2 g (20.8 mol) of terephthalic acid (TPA), 1536.1 g (24.8 mol) of ethylene glycol and 182.4 g (1.2 mol) of isosorbide (ISB) were placed in a 10 L reactor, A column that can be cooled with water and a condenser are connected to the reactor, but the content can be adjusted so that the content of the diol part derived from ISB introduced into the final polyester resin is 3 mol% relative to the total amount of the diol part . 1.0 g of germanium dioxide (the molar ratio of germanium dioxide to terephthalic acid is 1.25) was used as a catalyst, 1.46 g of phosphoric acid was used as a stabilizer, and 0.7 g of cobalt acetate was used as a colorant. Next, nitrogen was injected into the reactor to form a pressurized state, in which the pressure of the reactor was 1.0 kgf/cm ² higher than the normal pressure.

In addition, the temperature of the reactor was increased to 260°C and kept at the same temperature, and the esterification reaction was carried out until the mixture in the reactor became transparent. During this process, unreacted ISB and by-products flow out through the column and condenser. After the esterification reaction is completed, the nitrogen in the pressurized reactor is discharged to the outside to reduce the pressure of the reactor to normal pressure, and then the mixture in the reactor is transferred to a 7 L reactor capable of vacuum reaction.

Then, the temperature of the reactor was increased to 280° C. within 1 hour, and the polycondensation reaction was performed while maintaining the pressure of the reactor at 1 Torr or less. The polycondensation reaction proceeds until the intrinsic viscosity (melting IV) of the reaction product in the reactor becomes 0.55 dl/g. When the intrinsic viscosity of the reaction product reaches the desired level, the reaction product is discharged from the reactor and twisted. It is solidified with coolant and made into pellets so that the average weight is about 12 mg to 14 mg.

The particles were allowed to stand at 150°C for 1 hour in a nitrogen atmosphere for crystallization, and then put into a 20 L solid phase polymerization reactor. Then, nitrogen gas flows into the reactor at a rate of 50 L/min. Here, the temperature of the reactor was increased from room temperature to 140°C at a rate of 40°C/hour, and kept at 140°C for 3 hours. Thereafter, the temperature was further increased to 200°C at a rate of 40°C/hour and kept at 200°C. The solid phase polymerization reaction is carried out until the intrinsic viscosity (solid phase IV) of the particles in the reactor reaches 0.70 dl/g.

Comparative preparation example 2

Put 2988.9 g (18.0 mol) of terephthalic acid, 1228.0 g (19.8 mol) of ethylene glycol (EG) and 777.8 g (5.4 mol) of cyclohexane dimethanol (CHDM) into 10 L In the reactor, a water-cooled pipe string and a condenser are connected to the reactor, but the content is adjusted so that the content of the diol part derived from CHDM is introduced into the final polyester resin relative to the total amount of the diol part It is 30 mol%. 0.7 g of germanium dioxide (the molar ratio of germanium dioxide to terephthalic acid is 1.4) is used as a catalyst, 1.2 g of phosphoric acid is used as a stabilizer, and 0.5 g of cobalt acetate is used as a colorant. Then, nitrogen was injected into the reactor to form a pressurized state, in which the pressure of the reactor was 2.0 kgf/cm ² higher than the normal pressure.

In addition, the temperature of the reactor was increased to 255°C and kept at the same temperature to proceed the esterification reaction until the mixture in the reactor became transparent. During this process, unreacted ISB and by-products flow out through the column and condenser. After the esterification reaction is completed, the nitrogen in the pressurized reactor is discharged to the outside to reduce the pressure of the reactor to normal pressure, and then the mixture in the reactor is transferred to a 7 L reactor capable of vacuum reaction.

Then, the temperature of the reactor was increased to 275° C. within 1 hour, and the polycondensation reaction was performed while maintaining the pressure of the reactor at 1 Torr or lower. The polycondensation reaction proceeds until the intrinsic viscosity (melting IV) of the reaction product in the reactor becomes 0.80 dl/g. When the intrinsic viscosity of the reaction product reaches the desired level, the reaction product is discharged from the reactor and twisted. It is solidified with coolant and made into pellets so that the average weight is about 12 mg to 14 mg.

Comparative preparation example 3

Place 3060.8 g (18.4 mol) of terephthalic acid (TPA), 971.7 g (15.7 mol) of ethylene glycol (EG), and 1076.8 g (7.4 mol) of isosorbide (ISB) into 10 In the L reactor, the column and condenser that can be cooled with water are connected to the reactor, but the content is adjusted so that the amount of the diol part derived from the ISB in the final polyester resin is introduced relative to the total amount of the diol part. The content is 20 mol%. 1.0 g of germanium dioxide (the molar ratio of germanium dioxide to terephthalic acid is 1.25) as a catalyst, 1.46 g of phosphoric acid as a stabilizer, and 0.017 g of Polysynthrene Blue RLS (manufactured by Clarient) as a blue tint As the toner, 0.006 g of Solvaperm Red BB (manufactured by Clarient Corporation) was used as the red toner. Then, nitrogen was injected into the reactor to form a pressurized state, in which the pressure of the reactor was 2.0 kgf/cm ² higher than the normal pressure.

In addition, the temperature of the reactor was increased to 265°C and kept at the same temperature to proceed the esterification reaction until the mixture in the reactor became transparent. During this process, unreacted ISB and by-products flow out through the column and condenser. After the esterification reaction is completed, the nitrogen in the pressurized reactor is discharged to the outside to reduce the pressure of the reactor to normal pressure, and then the mixture in the reactor is transferred to a 7 L reactor capable of vacuum reaction.

Then, the temperature of the reactor was increased to 280° C. within 1 hour, and the polycondensation reaction was performed while maintaining the pressure of the reactor at 1 Torr or less. The polycondensation reaction proceeds until the intrinsic viscosity (melting IV) of the reaction product in the reactor becomes 0.60 dl/g. When the intrinsic viscosity of the reaction product reaches the desired level, the reaction product is discharged from the reactor and twisted. It is solidified with coolant and made into pellets so that the average weight is about 12 mg to 14 mg.

Comparative preparation example 4

Combine 3156.2 g (19.0 mol) of terephthalic acid (TPA), 730.9 g (11.8 mol) of ethylene glycol (EG), 684.5 g (4.8 mol) of cyclohexane dimethanol (CHDM) and 499.7 g (3.4 mol) of isosorbide (ISB) was placed in a 10 L reactor, and a water-cooled column and condenser were connected to the reactor, but the content was adjusted so that relative to the glycol part The total content of the diol part derived from ISB and the diol part derived from CHDM introduced into the final polyester resin is 10 mol% and 25 mol%, respectively. 1.0 g of germanium dioxide (the molar ratio of germanium dioxide to terephthalic acid is 1.05) is used as a catalyst, 1.46 g of phosphoric acid is used as a stabilizer, and 0.9 g of cobalt acetate is used as a colorant. Then, nitrogen was injected into the reactor to form a pressurized state, in which the pressure of the reactor was 1.0 kgf/cm ² higher than the normal pressure.

In addition, the temperature of the reactor was increased to 260°C and maintained at the same temperature to proceed the esterification reaction until the mixture in the reactor became transparent. During this process, unreacted ISB and by-products flow out through the column and condenser. After the esterification reaction is completed, the nitrogen in the pressurized reactor is discharged to the outside to reduce the pressure of the reactor to normal pressure, and then the mixture in the reactor is transferred to a 7 L reactor capable of vacuum reaction.

Then, the temperature of the reactor was increased to 270° C. within 1 hour, and the polycondensation reaction was performed while maintaining the pressure of the reactor at 1 Torr or lower. The polycondensation reaction proceeds until the intrinsic viscosity (melting IV) of the reaction product in the reactor becomes 0.70 dl/g. When the intrinsic viscosity of the reaction product reaches the desired level, the reaction product is discharged from the reactor and twisted. It is solidified with coolant and made into pellets so that the average weight is about 12 mg to 14 mg.

For the polyester resin prepared in the preparation example and the comparative preparation example, the content and intrinsic viscosity (IV) of the diol part derived from ISB and CHDM were measured. The results are shown in Table 1 below. Table 1 Preparation example Comparative preparation examples 1 2 3 4 5 6 1 2 3 4 Melting IV ¹⁾ (dl/g) 0.55 0.50 0.60 0.50 0.50 0.60 0.55 0.80 0.60 0.70 Solid phase IV ²⁾ (dl/g) 0.70 0.75 ND 0.75 0.75 ND 0.70 ND ND ND ISB content3 ⁾ (mol%) 5 10 16 10 10 10 3 0 20 10 CHDM content ⁴⁾ (mol%) 0 0 0 0 0 6 0 30 0 25

The term "ND" in Table 1 above means that since the crystallization and solid-phase polymerization reaction did not proceed after the polycondensation reaction in the preparation of the polyester resin, the solid-phase intrinsic viscosity (IV) was not measured.

In Table 1 above, the intrinsic viscosity (IV) and content of the glycol part are as follows:

1) Melting IV: The intrinsic viscosity of the reaction product obtained after the polycondensation reaction in the preparation of the polyester resin.

2) Solid phase IV: the intrinsic viscosity of the reaction product obtained through the crystallization and solid phase polymerization reaction after the polycondensation reaction in the preparation of the polyester resin.

3) ISB content: relative to the total amount of the diol part derived from all the diols contained in the final polyester resin being 100 mol%, the molar ratio of the diol part derived from isosorbide (ISB) .

4) CHDM content: relative to the total amount of the diol part derived from all diols contained in the final polyester resin being 100 mol%, the content of the diol part derived from cyclohexane dimethanol (CHDM) Ear ratio.

>Preparation of polyester film>

Example 1

Add the polyethylene terephthalate (SKYPE BL8050 grade, melting point of 255 ℃, manufactured by SK Chemicals) prepared in Preparation Example 1 and polyester resin into the extruder at a weight ratio of 70:30, and Melting at a temperature of 250-300°C.

Then, the polyester resin is extruded through a die to produce an unstretched polyester sheet. Subsequently, the unstretched polyester sheet is stretched at a stretching ratio of 1 time in the longitudinal direction and a stretching ratio of 1 time in the transverse direction, and then heat-set at 100°C to 220°C. Therefore, a polyester film with a thickness of 1 mm can be obtained.

Examples 2 to 5

A polyester film with a thickness of 1 mm was prepared in the same manner as in Example 1, except that the mixing of each resin and the stretching performed are shown in Table 2 below.

Examples 6 to 11

A polyester film with a thickness of 200 μm was prepared in the same manner as in Example 1, except that the mixing of each resin and the stretching performed are shown in Table 2 below.

Example 12

After adding the polyethylene terephthalate and polyester resin prepared in Preparation Example 3 into the extruder at a weight ratio of 65:35, the ratio of the polyethylene terephthalate and polyester resin In terms of the total weight, 200 ppm of polyethylene (prepared in the form of master batch (m/B)) is added to the extruder as a crystallization aid, and then at a temperature of 250-300°C Melting.

Then, the polyester resin is extruded through a die to produce an unstretched polyester sheet. Subsequently, the unstretched polyester sheet was stretched at a stretching ratio of 2.5 times in the longitudinal direction and a stretching ratio of 3 times in the transverse direction, followed by heat setting. Therefore, a polyester film with a thickness of 200 μm can be obtained.

Example 13

A polyester film with a thickness of 200 μm was prepared in the same manner as in Example 12, except for the mixing of each resin and each additive and the stretching performed as shown in Table 2 below.

Examples 14 and 15

A polyester film with a thickness of 200 μm was prepared in the same manner as in Example 1, except that the mixing of each resin and the stretching performed are shown in Table 2 below.

Example 16

A polyester film with a thickness of 1 mm was prepared in the same manner as in Example 1, except for the mixing of each resin and each additive and the stretching performed as shown in Table 2 below.

Example 17

A polyester film with a thickness of 200 μm was prepared in the same manner as in Example 1, except that the mixing of each resin and each additive and the stretching performed are shown in Table 2 below.

Comparative example 1

Only 100 parts by weight of polyethylene terephthalate (SKYPE BL8050 grade, manufactured by SK Chemicals) was added to the extruder and melted at a temperature of 250-300°C.

The polyester resin is then extruded through a die to produce an unstretched polyester sheet. Subsequently, the unstretched polyester sheet was stretched at a stretching ratio of 1 time in the longitudinal direction and a stretching ratio of 1 time in the transverse direction, followed by heat setting. Thus, a polyester film was obtained.

Comparative examples 2 to 10

The polyester film was prepared in the same manner as in Example 1, except that the mixing of each resin and the stretching performed are shown in Table 3 below. Table 2 Instance 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 Resin A PET PET PET PET PET PET PET PET PET PET PET PET PET PET PET PET PET Resin B Preparation example 1 Preparation example 2 Preparation example 3 Preparation example 3 Preparation example 2 Preparation example 1 Preparation example 2 Preparation example 3 Preparation example 2 Preparation example 2 Preparation example 3 Preparation example 3 Preparation example 3 Preparation example 4 Preparation Example 5 Preparation Example 6 Preparation Example 6 A:B blended weight ratio 70:30 80:20 90:10 10:90 30:70 50:50 80:20 10:90 30:70 60:40 85:15 63:35 63:35 80:20 80:20 30:70 30:70 Additives (content*) - - - - - - - - - - - Crystallizer (200ppm) Crystallizer (100ppm) - - - - Stretching process Longitudinal stretch ratio 1 1 1 1 1 1 2 3 2 3 2 2.5 2 2 3 1 2.5 Horizontal stretch ratio 1 1 1 1 1 5 3 2 3 4 3 3 3 2 3 1 2.5 Total stretch ratio 1 1 1 1 1 5 6 6 6 12 6 7.5 6 4 9 1 6.25 table 3 Comparative example 1 2 3 4 5 6 7 8 9 10 Resin layer A PET PET PET PET PET PET PET PET PET PET Resin layer B - Comparative Preparation Example 1 Comparative Preparation Example 2 Comparative Preparation Example 3 - Comparative Preparation Example 1 Comparative Preparation Example 4 Comparative Preparation Example 2 Preparation Example 3 Preparation Example 1 A:B blended weight ratio 100:0 80:20 80:20 80:20 100:0 75:25 80:20 80:20 5:95 95:5 Additives (content*) - - - - - - - - - - Stretching process Longitudinal stretch ratio 1 1 1 1 2 3 3 3 2 1 Transverse stretch ratio 1 1 1 1 3 4 4 4 3 1 Total stretch ratio 1 1 1 1 6 12 12 12 6 1

In Table 2 and Table 3 above, the content of the additives is calculated in units of weight relative to the total weight of resin A and resin B. In the case of Comparative Examples 7 and 8 in Table 3 above, the stretching process was attempted to be carried out at the above-mentioned stretching ratio, but the diol part derived from ISB and CHDM contained in the polyester resin has a high content, so that no The styling property is increased, so the stretch alignment cannot be performed.

Experimental example: Evaluation of physical properties of polyester stretch film

The physical properties of the polyester resins prepared in Examples 1 to 17 and Comparative Examples 1 to 10 were evaluated according to the above methods, and the evaluation results are shown in Tables 4 and 5. However, since the stretched films of Comparative Examples 7 and 8 were not prepared, they could not be evaluated based on their physical properties. Table 4 Instance 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 membrane thickness 1mm 1mm 1mm 1mm 1mm 200μm 200μm 200μm 200μm 200μm 200μm 200μm 200μm 200μm 200μm 1mm 200μm tan δ(℃) 88 90 97 97 95 112 115 112 110 120 120 121 122 115 115 95 95 strain(%) 2 3 3 3 3 2 2 3 3 1 2.5 2.5 2.5 2 2 3 3 Adhesion O O O O O O O O O O O O O O O O O Haze 3 2 1 0.5 0.5 2 2 0.5 0.5 1.5 1.5 2 1.5 2 2 0.5 0.5 table 5 Comparative example 1 2 3 4 5 6 7 8 9 10 membrane thickness 1mm 1mm 1mm 1mm 200μm 200μm 200μm 200μm 200μm 200μm tan δ(℃) 85 86 85 100 100 108 ND ND 97 110 strain(%) 2 2 10 15 2 2 ND ND 10 2 Adhesion X X O O X X ND ND O X Haze 10 5 20 15 2 5 ND ND 1 5

In Table 5, the term "N.D." means unable to measure.

From the above experimental results, it can be confirmed that the difference in physical properties of the polyester film of the example and the comparative example depends on the type of resin under the same thickness. It can also be confirmed that the polyester film of the example prepared by mixing the polyester resin (resin B) with polyethylene terephthalate at the optimal weight ratio has significantly improved transparency compared to the comparative example With respect to 100 mol% of the diol portion derived from diol in the polyester resin, the polyester resin (Resin B) contains 18 mol% or less derived from ISB And at least one diol moiety in CHDM, and 4 to 18 mol% of the diol moiety derived from ISB.

Specifically, from the results of Comparative Examples 1 and 5 prepared by using polyethylene terephthalate alone, it can be confirmed that although the heat resistance and haze characteristics can be further improved through the stretching process, the The diester has high crystallinity and cannot have adhesiveness.

In addition, it can be confirmed from the results of Comparative Examples 2, 6, and 10 that even if polyethylene terephthalate is mixed with a polyester resin satisfying the above conditions, when the content of the diol portion derived from ISB in the polyester resin is low Or when the content of the blended polyester resin is insufficient, the adhesion is still insufficient regardless of whether it is stretched or not.

Moreover, when the content of the diol part derived from the ISB in the polyester resin is higher or the content of the mixed polyester resin is sufficient, the desired degree of adhesion can be achieved due to the decrease in crystallinity. However, when the content of the diol moiety derived from ISB in the polyester resin is too high, as in Comparative Example 4, the strain may increase significantly and the haze characteristics may deteriorate. In addition, when the content of the mixed polyester resin is too high, as in Comparative Example 9, strain may increase.

In addition, referring to the result of Comparative Example 3, when the polyester resin does not include the diol portion derived from ISB, heat resistance and haze characteristics may decrease, and strain may also increase. From the results of Comparative Example 9, it can also be confirmed that the stretching process cannot be performed due to the increase in amorphousness.

With reference to the result of Comparative Example 7, even if the polyester resin includes the diol part derived from ISB, when the total content of the diol part derived from ISB and CHDM is too high, the increase in amorphousness may make the stretching process impossible.

From these results, it can be confirmed that in this disclosure, in order to improve heat resistance, adhesion, and transparency, the mixing ratio of polyester resin and polyethylene terephthalate and the ratio of polyester resin derived from ISB and CHDM should be controlled at the same time. The content conditions of the glycol part.

The polyester stretch film according to an embodiment of the present disclosure includes polyester resin and polyethylene terephthalate in an optimal mixing ratio to have heat resistance, transparency, and excellent adhesion, which can be controlled in poly The content of the diol moiety derived from ISB and CHDM in the ester resin. Therefore, polyester stretch films can be expected to be used in various applications, such as industrial films, food container films, packaging films, optical films, insulating films, printing films, and adhesive films.

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Claims (20)

A polyester film, including: A resin layer, formed by a mixture including polyethylene terephthalate and polyester resin, wherein the weight ratio of polyethylene terephthalate to polyester resin is 90:10 Until 10:90, The polyester resin has a repeating structure of an acid moiety derived from dicarboxylic acid or its derivatives and a diol moiety derived from diol, and the polyester resin Is characterized in that relative to the total amount of diol moieties derived from the diol as 100 mole percent (mol%), the content (a) of the first diol moiety derived from isosorbide and derived The content (b) of the second diol part from cyclohexanedimethanol satisfies the following formula 1: [Formula 1] b≤18 mol%-a, Wherein, in the formula 1, a is the content (mol%) of the first diol part derived from isosorbide, relative to the total amount of the diol part derived from the diol in the polyester resin is 100 In terms of mol%, a is 4 mol% to 18 mol%, and b is the content of the second diol part derived from cyclohexanedimethanol. The polyester film according to claim 1, wherein the dicarboxylic acid or its derivative comprises terephthalic acid or its derivative. The polyester film according to claim 1, wherein relative to the total amount of the dicarboxylic acid or its derivatives, the dicarboxylic acid or its derivatives includes less than or equal to 60 mol% of aromatic materials selected from C8 to C14 At least one of the group consisting of aromatic dicarboxylic acid (aromatic dicarboxylic acid) or its derivatives and C4 to C12 aliphatic dicarboxylic acid (aliphatic dicarboxylic acid) or its derivatives, as different from terephthalic acid ( terephthalic acid) or its derivative dicarboxylic acid or its derivative. The polyester film according to claim 1, wherein relative to the total amount of the diol part is 100 mol%, the diol part further comprises 82 mol% to 96 mol% of aliphatic two derived from C2 to C12 The third diol moiety of the carboxylic acid or its derivative. The polyester film according to claim 4, wherein, relative to the total amount of the diol part being 100 mol%, the diol part is composed of 4 mol% to 18 mol% of the first two diols derived from isosorbide The alcohol part is composed of 82 mol% to 96 mol% of the third glycol part derived from ethylene glycol. The polyester film according to claim 1, wherein based on a central metal atom, the polyester film includes a polycondensation catalyst selected from 1 ppm to 300 ppm, a phosphorus stabilizer from 10 ppm to 5000 ppm, Consists of 1 ppm to 300 ppm of a cobalt-based coloring agent, 1 ppm to 200 ppm of a crystal promoter, 10 ppm to 500 ppm of an antioxidant, and 10 ppm to 300 ppm of a branching agent At least one of the group of. The polyester film according to claim 1, wherein the polyester resin has an intrinsic viscosity of 0.50 deciliter/gram (dl/g) to 1.00 dl/g, and the intrinsic viscosity is such that the polyester resin is dissolved at 150°C The intrinsic viscosity of ortho-chlorophenol at a concentration of 1.2 grams per deciliter (g/dl) measured at 35°C after 15 minutes. The polyester film according to claim 1, wherein the polyethylene terephthalate has a melting point of 220°C to 260°C. The polyester film according to claim 1, wherein the polyethylene terephthalate has repeated acid moieties derived from dicarboxylic acids or derivatives thereof and glycol moieties derived from ethylene glycol Structure. The polyester film according to claim 1, wherein relative to a total weight of the polyethylene terephthalate and the polyester resin, the polyester film further includes 5 ppm to 200 ppm of a crystallization aid. The polyester film according to claim 1, wherein the polyester film is a film stretched in at least one of the longitudinal direction and the transverse direction. The polyester film according to claim 1, wherein the polyester film is a film biaxially stretched at a stretching ratio of 2 to 5 times in the longitudinal direction and a stretching ratio of 2 to 7 times in the transverse direction. The polyester film according to claim 1, wherein the polyester film is an unstretched film, and when the thickness is 1 millimeter (mm), the haze measured according to ASTM D1003-97 is 3% or more Less, the loss factor (tan δ) calculated according to the following formula 4 is 88°C or less, and the strain at 100°C calculated according to the following formula 5 is 3% or less, [Formula 4] tan δ=E’/E”, In this formula 4, E'and E" are the Young's modulus and loss modulus, respectively, and are measured by dynamic mechanical analysis under fixed frequency and temperature conditions. In this temperature condition, the temperature is 3°C/min. The rate of (min) increases from room temperature to 150°C, [Formula 5] Strain (%)=[(length of the polyester film after applying stress at 100°C-length of the polyester film before applying stress)/length of the polyester film before applying stress]×100, In this formula 5, the length of the polyester film after the stress is applied and the length of the polyester film before the stress are respectively calculated according to the Creep TTS test and the film length before the stress is applied. After the temperature of the polyester film was increased from room temperature, 10 MPa was applied to the film under isothermal conditions at 100° C. for a length of 10 minutes of deformation. The polyester film according to claim 1, wherein the polyester film is a stretched film, and when the thickness is 200 microns (μm), the haze measured according to ASTM D1003-97 is 3% or less , The tan δ calculated according to the following formula 4 is 110°C or more, and the strain at 100°C calculated according to the following formula 5 is 3% or less, [Formula 4] tan δ=E’/E”, In this formula 4, E'and E" are the Young's modulus and loss modulus, respectively, and are measured by dynamic mechanical analysis under fixed frequency and temperature conditions. In this temperature condition, the temperature is 3°C/min. The rate of (min) increases from room temperature to 150°C, [Formula 5] Strain (%)=[(length of the polyester film after applying stress at 100°C-length of the polyester film before applying stress)/length of the polyester film before applying stress]×100, In this formula 5, the length of the polyester film after the stress is applied and the length of the polyester film before the stress are respectively calculated according to the Creep TTS test and the film length before the stress is applied. After the temperature of the polyester film was increased from room temperature, 10 MPa was applied to the film under an isothermal condition of 100° C. for a length of 10 minutes of deformation. A method for preparing the polyester film according to claim 1, comprising: The polyethylene terephthalate and polyester resin are mixed in a weight ratio of 90:10 to 10:90, and then melt-extrusion is performed to prepare a mixture composed of polyethylene terephthalate and polyester resin. Resin layer, The polyester resin has a repeating structure of an acid moiety derived from dicarboxylic acid or its derivatives and a diol moiety derived from diol, and the polyester resin Is characterized by the content (a) of the first diol part derived from isosorbide and the first diol part derived from cyclohexanedimethanol relative to the total amount of the diol part derived from the diol being 100 mol% The content of the didiol part (b) satisfies the following formula 1: [Formula 1] b≤18 mol%-a, Wherein, in the formula 1, a is the content (mol%) of the first diol part derived from isosorbide, relative to the total amount of the diol part derived from the diol in the polyester resin is 100 In terms of mol%, a is 4 mol% to 18 mol%, and b is the content of the second diol part derived from cyclohexanedimethanol. The method for producing a polyester film according to claim 15, further comprising stretching the polyester film in at least one of the longitudinal direction and the transverse direction after the preparation of the polyester film. The method for producing a polyester film according to claim 16, wherein the stretching is performed by biaxially stretching the polyester film at a stretching ratio of 2 to 5 times in the longitudinal direction and a stretching ratio of 2 to 7 times in the transverse direction. Stretch out. The preparation method of the polyester film according to claim 16, further comprising heat setting the stretched polyester film at 100°C to 220°C after the stretching. The preparation method of polyester film as described in claim 15 further includes: Before mixing polyethylene terephthalate and polyester resin in the step of preparing the polyester film, (a0-1) Esterification or transesterification of dicarboxylic acid or its derivatives and diol; and (a0-2) The product obtained by the esterification reaction or the transesterification reaction is subjected to a polycondensation reaction to prepare a polyester having an intrinsic viscosity of 0.50 deciliter/gram (dl/g) to 1.00 dl/g The intrinsic viscosity is the intrinsic viscosity of the polyester resin dissolved in ortho-chlorophenol at a concentration of 1.2 g/dl (g/dl) at 150°C for 15 minutes at 35°C. The preparation method of the polyester film as described in claim 19 further includes: Before step (a0-2) and before step (a), (a0-3) crystallize the polymer obtained by the polycondensation reaction, (a0-4) The crystalline polymer is subjected to solid phase polymerization to have the intrinsic viscosity of 0.10 dl/g to 0.40 dl/g, wherein the intrinsic viscosity is that the polyester resin is dissolved at a concentration of 1.2 g at 150°C / Deciliter (g/dl) of ortho-chlorophenol (ortho-chlorophenol) after 15 minutes at 35 ℃ intrinsic viscosity, and the intrinsic viscosity is higher than that of the polymer obtained in step (a0-2) Viscosity.